The present invention relates to an extraction method for recovering aromatic bisimide from an aromatic bisimide reaction mixture. An organic solvent solution of aromatic bisimide reaction solids is extracted with water and/or extracted with an aqueous alkali metal hydroxide.
Prior to the present invention, a method for making aromatic bisimides of the formula, ##STR1## was based on the reaction of a substituted N-organo phthalimide and an alkali bisphenoxide in a dipolar aprotic solvent, where R is a monovalent C.sub.(1-13) organic radical and Z is a C.sub.(6-30) divalent aromatic organic radical. The resulting reaction mixture contained in addition to the aromatic bisimide of formula (1), a variety of reaction solids including phenolic monoimide of the formula, ##STR2## alkali metal substituted phthalic salts of the formula, ##STR3## where X is selected from a nitro or halo radical and M is an ion of an alkali metal, unreacted substituted N-alkyl phthalimide of the formula, ##STR4## and alkali metal bisphenoxide salts of the formula, EQU Z--OM).sub.2. (5)
in addition, there also was included in the reaction mixture alkali metal nitrites and substituted phthalic acid amides of the formula, ##STR5## where R and Z are as previously defined.
Radicals included by Z of formula (1), are for example, ##STR6## and divalent organic radicals of the general formula, ##STR7## where Q is a member selected from the class consisting of divalent radicals of the formulas, ##STR8## and --S-- and y is an integer from 1 to 5. Monovalent organic radicals included within R are, for example, C.sub.(6-13) aromatic hydrocarbon radicals, C.sub.(1-8) aliphatic radicals and halogenated derivatives thereof, for example, methyl, ethyl, propyl, heptyl, octyl, chloroethyl, etc.; phenyl, tolyl, xylyl, naphthyl, anthryl, etc.; cycloalkylene radicals having from 3 to about 12 carbon atoms, etc.
M of formula (2) can include alkali metal ions, for example, sodium, potassium, etc., and X is a monovalent radical selected from nitro, and halo radicals, for example, chloro, fluoro, bromo, etc.
The procedures of the prior art used to isolate the aromatic bisimide of formula (1) from the aforementioned impurities were generally based on recovering the reaction mixture as a crystalline solid and further treating the solid with an appropriate solvent, grinding the solvent-solid mixture, filtering, further grinding with solvent, etc.
As shown in copending applications Ser. Nos. 37,441 and 37,442 of Frank J. Williams et al filed May 9, 1979, improved solids handling results can be achieved with respect to recovering the aromatic bisimide of formula (1) in substantially pure form by substituting a non-polar organic solvent, such as toluene, in combination with a phase transfer catalyst for the dipolar aprotic solvent of the prior art. Unlike the solid handling procedures required in isolating the aromatic bisimide when using a dipolar aprotic solvent, the phase transfer catalyst allows for the production of a non-polar organic solvent solution of the aromatic bisimide and other reaction solids which can be readily separated therefrom with a precipitating organic solvent such as methanol. However, experience has shown that such precipitating solvents often do not satisfactorily remove particular reaction components, such as the unreacted N-alkyl-nitrophthalimide of formula (4). In addition, the use of such precipitating solvents are often uneconomic and can have an adverse environmental impact.
The present invention is based on the discovery that aromatic bisimide of formula (1), can be recovered free of contaminating solids of formulas (2-6), without an extensive solids handling procedure, by treating a solution of the aromatic bisimide reaction mixture with water and/or an aqueous solution of an alkali metal hydroxide. In the event a water soluble dipolar aprotic organic solvent is initially used in the reaction mixture, a substantially water insoluble organic solvent, for example, methylene chloride, can be used prior to the aqueous wash and/or aqueous base treatment to prevent premature separation of reaction solids.